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This lesson covers translation — the second stage of gene expression in which the nucleotide sequence of mRNA is decoded to produce a polypeptide. Understanding translation is essential for the Edexcel A-Level Biology specification (9BI0, Topic 7).
Translation is the process by which the sequence of codons on mRNA is decoded by ribosomes and transfer RNA (tRNA) to assemble a chain of amino acids — a polypeptide. Translation takes place on ribosomes in the cytoplasm (or on the rough endoplasmic reticulum for secreted proteins).
Ribosomes are the molecular machines that carry out translation. They consist of two subunits made of ribosomal RNA (rRNA) and proteins:
| Feature | Eukaryotic ribosome | Prokaryotic ribosome |
|---|---|---|
| Size | 80S | 70S |
| Large subunit | 60S | 50S |
| Small subunit | 40S | 30S |
Each ribosome has three binding sites for tRNA:
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